The present invention relates to an electronic fuel injection system (EFI system) in an internal combustion engine.
In the EFI system, fuel is injected into the intake pipe of the engine through an injector, the injection timing being controlled so that it is synchronized with the engine revolution. Two types of EFI system are known. One is called a "speed density type" or "D-Jetronic type (D-J type)" EFI system. The other is called an "intake air flow sensing type" or "L-Jetronic type (L-J type)" EFI system.
In the D-J type EFI system, a fuel control circuit calculates the intake air quantity per engine revolution on the basis of the pressure in the intake manifold and the engine revolutional speed. Then, the circuit further calculates the optimum fuel injection quantity on the basis of the intake air quantity taking into account the engine conditions. The open time of the solenoid valve type injector is controlled by the circuit so that the calculated amount of fuel is supplied through the injector. If the engine revolutional speed is constant, the intake air quantity W is proportioned to the pressure B in the intake pipe, as shown in FIG. 5. The fuel control circuit has a memory circuit which memorizes the basic fuel injection quantity which corresponds to the intake air quantity which is proportional to the pressure in the intake manifold. The fuel control circuit determines the final fuel injection quantity by compensating the basic fuel injection quantity taking into account the engine revolutional speed. However, in the low revolutional speed range of the engine, the intake air quantity is greatly varied in response to the driving conditions of the engine or circumstances. Therefore, in the D-J type EFI system, the air-fuel ratio cannot be accurately controlled in the low revolutional speed range of the engine.
In the L-J type EFI system, the fuel control circuit calculates the optimum fuel injection quantity in response to the engine driving conditions on the basis of the output signals of an intake air flow meter and an engine speed sensor. The open time of the solenoid valve type injector is controlled by the circuit so that the calculated amount of fuel is supplied through the injector. Said intake air flow meter comprises: a dynamic pressure measuring plate rotatably arranged on the air flow passage in the intake pipe; and a spring which forces said plate against the air flow. The air flow quantity is detected by measuring the displacement of said plate caused by the dynamic pressure of the intake air of the engine. The intake air quantity greatly varies in response to the throttle valve opening, e.g., the quantity at full throttle opening is about twenty times that at idle operation of the engine. The air flow meter cannot accurately measure the intake air quantity throughout the entire range of the throttle valve opening because the quantity varies so much in response to the throttle valve opening as mentioned above. Besides, the output power of the engine is lost due to the increase of the intake air flow resistance because of the dynamic pressure measuring plate of the air flow meter, which is arranged against the intake air flow in the intake pipe.